Polarization conversion system and method for projecting polarization encoded imagery

ABSTRACT

A polarization conversion system separates light from an unpolarized image source into a first state of polarization (SOP) and an orthogonal second SOP, and directs the polarized light on first and second light paths. The SOP of light on only one of the light paths is transformed to an orthogonal state such that both light paths have the same SOP. A polarization modulator temporally modulates the light on the first and second light paths to first and second output states of polarization. First and second projection lenses direct light on the first and second light paths toward a projection screen to from substantially overlapping polarization encoded images. The polarization modulator may be located before or after the projection lenses. The polarization-encoded images may be viewed using eyewear with appropriate polarization filters.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application relates and claims priority to provisionalpatent application 60/916,970, entitled “Polarization conversion systemfor 3-D projection,” filed May 9, 2007; this patent application alsorelates and claims priority to provisional patent application60/988,929, entitled “Polarization conversion system for 3-Dprojection,” filed Nov. 19, 2007; and this patent application furtherrelates and claims priority to provisional patent application61/028,476, entitled “Polarization conversion system for stereoscopicprojection,” filed Feb. 13, 2008, all of which are herein incorporatedby reference for all purposes.

BACKGROUND

The disclosed embodiments relate generally to projection ofpolarization-encoded images and, more specifically, to a polarizationconversion system and method for transmitting polarization-encodedimagery to a projection screen.

FIG. 1 is a schematic diagram illustrating an exemplarypolarization-preserving display system 100. The display system 100includes a projection screen 102 and polarization filtering eyewear 104.Stereoscopic three-dimensional (3D) imagery is observed using a singlepolarization-preserving screen 102 sequentially displaying left andright perspective imagery, with polarization filtering eyewear 104. Thepolarization filtering eyewear 104 contains two lenses 106 and 108 ofalternately orthogonal polarization.

3D imagery can be synthesized using polarization control at theprojector to encode, and polarization filtering eyewear to decode theleft and right perspective imagery (See, e.g., commonly-owned U.S. Pat.No. 4,792,850, entitled “Method and system employing a push-pull liquidcrystal modulator,” to Lenny Lipton et al. and U.S. patent applicationSer. No. 11/424,087 entitled “Achromatic Polarization Switches,” filedJun. 14, 2006, both of which are herein incorporated by reference intheir entirety for all purposes).

A conventional implementation of polarization control after theprojection lens is shown in FIG. 2. Nearly-parallel rays emerge from theoutput of the lens, appearing to originate from a pupil inside of thelens, and converge to form spots on a distant screen. Ray bundles A, B,and C in FIG. 2 are bundles forming spots at the bottom, center, and topof a projection screen. The light emerging from the projection lens israndomly polarized, depicted in FIG. 2 as both S-and P-polarized light.The light passes through a linear polarizer, resulting in a singlepolarization state after the polarizer. The orthogonal polarizationstate is absorbed (or reflected), and the light flux after the polarizeris less than 50% of the original flux (resulting in a dimmer finalimage). The polarization switch is synchronized with the image frame,and the polarization state emerging from the polarization switch isalternated, producing images of alternately orthogonal polarization atthe screen. Polarization selective eyewear 104 allows images of onepolarization to pass to the left eye, and image of the orthogonalpolarization to pass to the right eye. By presenting different images toeach eye, 3D imagery can be synthesized.

This system is currently in use in movie theatres. However, typically,this system design suffers from having more than 50% of the lightabsorbed by the polarizer, and thus the resulting image is typicallymore than 50% dimmer than that of a typical 2D theatre. Moreover,time-sequential stereoscopic 3D further reduces the brightness by morethan 50%. The dimmer image can therefore limit the size of the theatreused for 3D applications and/or provides a less desirable viewingexperience for the audience.

SUMMARY

The present disclosure addresses the aforementioned issues as well asothers to provide a polarization conversion system and method forstereoscopic projection. Generally, a polarization conversion systemseparates light from an unpolarized image source into a first state ofpolarization (SOP) and an orthogonal second SOP, and directs thepolarized light on first and second light paths. The SOP of light ononly one of the light paths is transformed to an orthogonal state suchthat both light paths have the same SOP. A polarization modulatortemporally modulates the light on the first and second light paths tofirst and second output states of polarization. First and secondprojection lenses direct light on the first and second light pathstoward a projection screen to form substantially overlappingpolarization encoded images, much brighter than the referenced prior artsystem. The polarization-encoded images may be viewed using eyewear withappropriate polarization filters.

According to an aspect, a polarization conversion system fortransmitting polarization encoded imagery to a projection screenincludes a first projection lens, a second projection lens, apolarization splitter (PBS), a reflecting element, and a polarizationmodulator. The PBS is operable to transmit light of a first polarizationstate toward the first projection lens on a first light path, and isfurther operable to reflect light of a second polarization state towarda second light path. The reflecting element is located on the secondlight path and is operable to reflect light toward the second projectionlens. The polarization modulator may be located on the first and secondlight paths. The first and second projection lenses are operable todirect the polarization encoded images toward the projection screen.

In some embodiments, the polarization modulator may be a single unitthat is located on both the first and second light paths. In otherembodiments, the polarization modulator may include a first polarizationswitch and a second polarization switch, each polarization switch beinglocated on respective first and second light paths. The polarizationswitch(es) may be located before or after the projection lenses.

According to another aspect, a method for projectingpolarization-encoded stereoscopic image includes receiving unpolarizedimage source light at a polarization beam splitter. The method includestransmitting image source light of a first polarization state at thepolarization beam splitter toward a projection lens located on a firstlight path. The method also includes reflecting image light of a secondpolarization state at the polarization beam splitter toward a secondlight path. The method further includes reflecting light on the secondlight path toward a projection lens. The method additionally includesrotating the state of polarization of light on one of the first andsecond light paths light to an orthogonal state of polarization. Themethod further includes temporally modulating the state of polarizationof the light on the first and second light paths between a firstpolarized output state and a second polarized output state.

Other features will be apparent with reference to the foregoingspecification.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanyingfigures, in which like reference numbers indicate similar parts, and inwhich:

FIG. 1 is a schematic diagram illustrating an exemplarypolarization-preserving display system, in accordance with the presentdisclosure;

FIG. 2 illustrates a known implementation of polarization control in acinematic 3D system utilizing a polarization switch;

FIG. 3 is a schematic diagram illustrating a first embodiment of apolarization conversion system (PCS), in accordance with the presentdisclosure;

FIG. 3B is a schematic diagram illustrating a polarization convertingand switching module, in accordance with the present disclosure;

FIG. 4 is a schematic diagram illustrating a second embodiment of a PCS,in accordance with the present disclosure;

FIG. 5 is a schematic diagram illustrating a third embodiment of a PCS,in accordance with the present disclosure;

FIG. 6 is a schematic diagram illustrating a fourth embodiment of a PCS,in accordance with the present disclosure;

FIG. 7 is a schematic diagram illustrating a fifth embodiment of a PCS,in accordance with the present disclosure;

FIG. 8 is a schematic diagram illustrating a sixth embodiment of a PCS,in accordance with the present disclosure;

FIG. 9 is a schematic diagram illustrating a seventh embodiment of aPCS, in accordance with the present disclosure;

FIG. 10 is a schematic diagram illustrating an eighth embodiment of aPCS, in accordance with the present disclosure;

FIG. 11 is a schematic diagram of a ninth embodiment of a PCS, inaccordance with the present disclosure; and

FIG. 12 is a schematic diagram of a tenth embodiment of a PCS, inaccordance with the present disclosure.

DETAILED DESCRIPTION First Embodiment

FIG. 3 is a schematic diagram illustrating a first embodiment of apolarization conversion system (PCS) 300. Generally, PCS 300 may includean image source 304 (e.g., from a light modulating panel or conventionalfilm), initial relay lens 302, polarizing beam splitter (PBS) 310, firstand second relay lenses 306, 308, polarization switch 312, fold mirror318, polarization converting and switching module 320, and first andsecond projection lenses 328, 330, arranged as shown. As illustrated byFIG. 3B, polarization converting and switching module 320 may includepolarization converter 322 and polarization switch 324, and mayoptionally include a pre-polarizer 326 to improve contrast, all arrangedas shown. Polarization converter 322 is an optical component that isoperable to transform an input state of polarization to an orthogonalstate of polarization (e.g., a half wave plate).

The first and second relay lenses 306 and 308 are preferably symmetricabout respective aperture stops 301, 303, respectively located after thepolarization switch 312 and polarization converting and switching module320, providing substantially distortion-less images of the panel 304 ateach image location 314 and 316. In an alternative embodiment, theaperture stops 301, 303, may be located on the respective light paths305, 307, immediately prior to the polarization switch 312 andpolarization converting and switching module 320. In another alternativeembodiment, FIG. 3 depicts an alternate location 332 for thepolarization switch 312 in the first light path 306, and an alternatelocation 334 for the polarization converting and switching module 320 inthe second light path 308. These locations may prove to be beneficialalternatives if birefringence through the ions elements 302 of the relaysystem 300 degrades the system contrast. As another alternativelocation, the polarization switches 312, 324 may instead be placed afterthe projection lens rather than prior to it. Such an embodiment mayprovide system contrast advantages. Note that it is not necessary thatthe half wave plate 322 is immediately adjacent the polarization switch324—the half wave plate 322 may be located anywhere in the light pathbetween the PBS 310 and the polarization switch 324. Indeed, inalternative embodiments, the positions of the polarization switch 312and the polarization converting and switching module 320 may be reversedsuch that the polarization switch 312 is located on the second lightpath 307 and the polarization converting and switching module 320 islocated on the first light path 305.

In operation, panel 304 (e.g., a Digital Light Processing (DLP) panelfrom Texas Instruments or conventional film) is illuminated withrandomly polarized light from a light source (not shown) to provideunpolarized image source light. The light source may be, for example, aconventional UHP lamp, a xenon lamp, a light emitting diode lightsource, or in some embodiments, a light source taught in commonly-ownedU.S. patent application Ser. No. 11/779,708, entitled “Light collectorfor projection systems,” filed Jul. 18, 2007, herein incorporated byreference. The unpolarized image source light from the panel 304 isdirected toward PBS 310 by initial relay lens 302. The PBS 310 maytransmit P-polarized light on a first light path 305, and reflectS-polarized light toward a second light path 307. On the first lightpath 305, the P-polarized light passes through the polarization switch312, which operates to rotate the light passing through the switch 312in alternating frames, similar to the bundles A, B, and C in FIG. 2.

On the second light path 307, the S-polarized light reflected by the PBS310 passes to a fold mirror 318 (or any optical component that serves toreflect light without changing the polarization state, e.g., a prism).The S-polarized light then passes through a polarization converting andswitching module 320. The polarization converter 322 (which may be ahalf wave plate) preferably transforms substantially all visiblewavelengths to the orthogonal polarization (in this case, from S- toP-polarized light). The now-P-polarized light then passes throughpolarization switch 324. In some embodiments, a pre-polarizer 326 may beadded before or after module 320 for higher contrast. The polarizationswitch 324 included in the polarization and switching module 320operates to create alternating orthogonal states in a mannersubstantially identical to the switch 312 in the first light path 305.

The polarization conversion system 300 may form two separate images 314and 316 of the panel 304, each with magnification 1× (i.e., the outputimages at 314 and 316 may be substantially the same size as the inputimage from panel 304). It should be appreciated that the magnificationcould be other than 1× in this and other embodiments and that thismagnification is provided as an example. First and second projectionlenses 328 and 330 respectively image the intermediate images 314 and316 onto the projection screen 102. The projection lenses 328 and 330are allowed to move laterally, such that the images on the screen 102from the two optical paths 305 and 307 are superimposed, substantiallyoverlapping, preferably with minimal keystone distortion. Since nearlyall of the randomly polarized light from the panel 304 is imaged at thescreen 102 with a single polarization state, the resulting image of thesystem in FIG. 3 is approximately two times brighter than the image atthe screen 102 for the system in FIG. 2.

This system may also be applied to cinematic, professional and consumerapplications such as home theatre and rear projection television (RPTV),assuming polarization-preserving screens 102 are utilized.

Second Embodiment

FIG. 4 is a schematic diagram illustrating a second embodiment of apolarization conversion system (PCS) 400. PCS 400 provides a similarrelay system to that shown in FIG. 3, with an arrangement of componentshaving substantially similar structure and function, except a glassprism 410 has been inserted into the second light path 407, arranged asshown. Glass prism 410 may be a high index glass prism.

In operation, the glass prism 410 allows the two images 414 and 416 ofthe panel 404 to be collocated substantially in a single plane,providing more convenient packaging and adjustment of the projectionlenses 428 and 430. It is preferable that the relay system 400 isdesigned such that rays from a single field point at the object (i.e.,panel 404) produce a collimated bundle (all rays from a field pointhaving the same angle) at the aperture stops 401 and 403. This allowsthe insertion of the glass prism 410 at the aperture stop withoutaffecting the lens 402 performance. The glass prism 410 allows the twoimages 414 and 46 to be collocated. Again, in alternate embodiments, thepolarization converting and switching module 420 and polarization switch412 may each have alternate locations 404 and 406 respectfully for eachpath, either before the projection lens or after the projection lens.

Third Embodiment

FIG. 5 is a schematic diagram illustrating a third embodiment of apolarization conversion system (PCS) 500. FIG. 5 provides a similar PCS500 to that shown in FIG. 4, except the polarization switch 412 of FIG.4 has been replaced by a spinning wheel 550 operable to convert thepolarized input to a set of temporally alternating orthogonallypolarized output states. In one embodiment, the spinning wheel 550 maycontain segments that transmit alternating orthogonal polarization froma non-polarized input. In another embodiment, the spinning wheel 550 maybe preceded by a fixed polarizer. The spinning wheel 550 may thencontain segments that represent unitary polarization transformations(e.g. from a stack of retardation films).

An issue resulting from physical rotation of a polarizer (spinning wheel550) is that the output varies in an analog fashion, unless each segmentis patterned to compensate for this effect. Functionally, a binarypolarization switching effect is desired, which according to thisdisclosure is optimally accomplished using elements with circularEigenpolarizations. For instance, a true circular polarizer (versus, forexample, a linear polarizer followed by a retarder, or retarder stack)will transmit a particular handedness (e.g., right or left) of circularstate, regardless of wheel orientation.

Alternatively, a fixed polarizer can be followed by a unitarypolarization transforming element with circular Eigenpolarizations, or apure circular retarder. For instance, a linear polarizer can be followedby a rotating wheel 550 that contains a combination of isotropicsegments, as well as pure achromatic polarization rotating elements. Apure achromatic rotator has zero linear retardation (no optic axis), buthas a desired amount of phase delay between orthogonal circular states.In this case, a π phase shift between circular Eigenstates will convertthe input to the orthogonal linear output, regardless of wheelorientation. Thus, an analog wheel will provide binary switching betweenorthogonal linear polarizations.

Pure achromatic polarization rotators may be fabricated using stacks oflinear retarders. One design method is to pair stacks with a particularsymmetry arrangement. For instance, a stack that produces a particularretardation and rotation can be paired with an identical stack withreverse-order, or reverse-order reflected symmetry (See, e.g., Chapter 5of Robinson et. al., Polarization Engineering for LCD Projection, Wiley& Sons 2005, which is hereby incorporated by reference). A reverse orderstack doubles the net retardation while eliminating rotation, while theaddition of reflection has the effect of doubling rotation whileeliminating retardation. A stack designed to convert a 0-oriented linearinput to a π/4 oriented linear output (at all wavelengths of interest)can be designed, which in general contains linear retardation. However,when paired with the reverse-order-reflected stack, the net effect iszero retardation and the desired π/2 orientation transformation. Suchtransparent elements can be laminated as segments on an isotropic diskto produce binary polarization switching with spinning wheel 550.

Table 1 provides a design for an exemplary retarder stack exhibitingsubstantially achromatic rotation of π/2 having reverse-order-reflectedsymmetry. Note that this symmetry is a sufficient, but not necessarycondition for achieving the desired polarization transformation. It iseasily verified that the state of polarization after layer-6 is 45°linear, though the stack possesses linear retardation that is eliminatedby the subsequent stack. In this example, all layers have a zero-orderin-plane retardation of ½-wave (typically 240-270 nm to span thevisible). It should be apparent that, in accordance with the presentdisclosure, other retarder combination designs may be employed that havedifferent orientations and retardation profiles.

TABLE 1 Layer Number Orientation 1 −19.6° 2 2.4° 3 18.1° 4 −65.6° 5−54.3° 6 −15.0° 7 15.0° 8 54.3° 9 65.6° 10 −18.1° 11 −2.4° 12 19.6°

Still referring to FIG. 5, in operation, light from the lower light path505 is P-polarized and passes through the isotropic segment 504 of thewheel 550. The light remains P-polarized through Image 2 516, throughthe projection lens 530, and onto the screen 102. In this instance,light in the upper path 507 is S-polarized and passes through therotator segment 506 of the wheel 550. The S-polarized light is rotatedto P-polarized light by the wheel 550, and passes through the projectionlens 528 and onto the screen 102 as P-polarized light. The wheel 550 isthen synchronized with the video frames to produce images on a screen102 with alternating polarization. Variations of the polarization statesare also possible, with each path producing circular polarization byaddition of quarter-wave plates in the optical, paths, or rotated linearpolarization states (e.g., +/−45 degrees) by addition of rotators ineach path.

Fourth Embodiment

FIG. 6 is a schematic diagram illustrating a fourth embodiment of apolarization conversion system (PCS) 600. FIG. 6 illustrates a PCS 600where the magnification has been increase to 2× (versus 1× previously).In this case, the first half of the PCS 600, including PBS 610 andpath-matching glass prism 602 may be identical in structure and functionto the components described in FIG. 4. However, the second half of thePCS 600 has been scaled by 2 to increase the focal length of the secondhalf by 2. The PCS 600 produces an image that is twice the size of thepanel 604, yet maintains the same f-number (or numerical aperture). Inthis exemplary embodiment, a single relay lens 608 may be used toprovide an intermediate image, and a single projection lens 630 (e.g., a70 mm cinema lens) may be utilized to image the intermediate image tothe screen 102. The polarization converting and switching module 620 andits alternate location 625 are also shown.

Fifth Embodiment

FIG. 7 is a schematic diagram illustrating a fifth embodiment of a PSC700. FIG. 7 depicts a similar PCS 700 to that shown in FIG. 6, exceptthe polarization converting and switching modules 620 of FIG. 6 havebeen replaced by the segmented wheel 730 (similar to the segmented wheel550 described in FIG. 5). The segmented wheel 750 and an alternatesegment wheel location 752 are also indicated. Once again, a singleprojection lens 730 can be utilized to image the intermediate image tothe screen 102.

Sixth Embodiment

FIG. 8 is a schematic diagram illustrating a sixth embodiment of a PCS800. The PCS 800 may include a panel 804, an initial relay lens 802, aPBS 810, a polarization switch 812 on a first light path, a glass prism814 with a reflector (e.g., a mirrored angled surface) 816, apolarization converting and switching module 818, and first and secondprojection lenses 820 and 822, all arranged as shown. Polarizationconverting and switching module 818 may have an optional pre-polarizer,a polarization rotator and a polarization switch, similar to thedescription of the polarization converting and switching module 320 ofFIG. 3B. The projection lens system 800 may form two separate images ofthe panel 304, each with large magnification. This PCS 800 may also beapplied to professional and consumer applications such as home theatreand RPTV, assuming polarization-preserving screens 102 are available.

In operation, panel 804 (such as a Digital Light Processing, or DLP,panel from Texas Instruments) is illuminated with randomly polarizedlight. In this embodiment, light from the panel 804 is projected so ascreen 102 by first and second projection lenses 820 and 822, which maybe of the reverse telephoto type. The PBS 810 transmits P-polarizedlight along a first light path, and reflects S-polarized light along asecond light path. The P-polarized light passes through the polarizationswitch 812 and is rotated by the polarization switch 812 in alternatingframes, similar to bundles A, B, and C in FIG. 2.

The S-polarized light reflected by the PBS 810 (on the second lightpath) passes to a prism 814. The prism 814 may contain an angled surface816 that serves as a fold mirror. Reflection may be accomplished withtotal internal reflection, or by coating the hypotenuse with a mirrorlayer (e.g., silver). In order to insert such a prism 814 internal tothe PCS 800 without creating excessive aberrations in the final image,it is preferable that rays from a field point at the object (panel 304)are collimated (i.e., the rays in the bundle have the same angle) at theaperture stop(s) 830 and 832. In some embodiments, the aperture stop 830may be located along the first light path before the polarization switch812, and/or along the second light path at some location (i.e., 832)before the prism structure 814. Thus, collimated rays pass through theprism structure 814 without the introduction of aberrations. TheS-polarized light then passes out of the prism 814, through polarizationconverting and switching module 818, and is rotated to P-polarizedlight. The polarization switch within polarization converting andswitching module 818 acts on P-polarized light, rotating thepolarization of the ray bundles in alternating frames, insynchronization with the rotation of bundle sin the non-mirror path.

Two substantially identical second halves of the lenses 820 and 822project the two images onto the screen 102. To overlap the two images onthe screen 102, the polarizing beam splitter 810 tilt may be adjustedand/or the prism 808 tilt may be adjusted. The projection lens assembly,may as a whole, be allowed to move laterally, such that the images onthe screen 102 from the first and second optical paths can be offsetvertically for various theatre configuration. The first half lenses 820may be cut in the lower path to allow for light to pass clearly in theupper path, as is depicted in FIG. 8.

Since nearly all of the randomly polarized light from the panel 804 isimaged at the screen 102 with a single polarization state, the resultingimage of the system in FIG. 8 is approximately two times brighter thanthe image at the screen 102 for the system in FIG. 2.

Seventh Embodiment

FIG. 9 depicts a similar polarization conversion system 900 as in FIG.8, except that the polarization switch 812 has been replaced by aspinning wheel 902. The wheel 902 is segmented into two of more regionsas described previously. In this instance, light from the lower path 904is P-polarized and passes through the (e.g.) isotropic segment 901 ofthe wheel 902. The light remains P-polarized through the rest of theprojection lens system 900, and onto the screen 102. In this instance,light in the upper path 906 is S-polarized and passes through the (e.g.)rotator segment 903 of the wheel 902. The S-polarized light is rotatedto P-polarized light by the wheel 902, and passes through the rest ofthe projection system 900 and onto the screen 102 as P-polarized light.The wheel 902 is then synchronized with the video frames to produceimages on screen 102 with alternating polarization variations of thepolarization states are also possible, with each path 904 and 906producing circular polarization by addition of quarter wave plates (notshown) in the optical paths, or rotated linear polarization states(e.g., +/−45 degrees) by addition of rotators in each path.

Eighth Embodiment

FIG. 10 depicts a similar polarization conversion system 1000 to that ofFIG. 9. In this exemplary embodiment, the structure and function of thecomponents of the PCS 1000 are substantially similar to that of the PCS900, except two rotator wheels 1002 and 1004 are implemented instead ofone, in part, to ease packaging constraints near the prism 808. Therotator wheels 1002 and 1004 may operate in synchronization with eachother.

Ninth Embodiment

FIG. 11 is a schematic diagram of an exemplary cinematic PCS system 1100that implements zoom lenses. Cinematic PCS system 1100 may include apanel 1102, a telecentric objective 1104 (i.e., an initial relay lens),a polarization beam splitter (PBS) 1106, first and second aperture stops1108, 1110, first and second mechanically compensated afocal zooms 1112,1132, reflecting element 1130, rotator 1136, and first and secondz-screens 1124, 1138.

In operation, s- and p-polarized light from panel 1102 passes throughtelecentric objective 1104 toward PBS 1106. Telecentric objective 1104is used to maintain collimated light at the PBS 1106 for all zoomsettings. PBS 1106 may be a cube or wire grid plate, or any other PBSknown in the art. In this embodiment, p-polarized light is transmittedthrough the PBS 1106 toward a first direction, while s-polarized lightis reflected at the PBS 1106 toward a second direction.

The p-polarized light passes through aperture stop 1108 toward a firstmechanically compensated afocal zoom apparatus 1112. Zoom 1112 mayinclude various elements having positive and negative optical powers.The afocal zoom can be mechanically compensated or opticallycompensated, for instance, using techniques in zoom lens design from“Modern Optical Engineering” by Warren Smith, 1990, McGraw-Hill, hereinincorporated by reference. Zoom 1112 in this exemplary embodiment mayhave, on a light path, a fixed optical element such as concave lens1114, followed by moving elements convex lens 1116 and concave lens1118, followed by another fixed element, convex lens 1120. Generally inFIG. 11, convex lenses are represented by lines with dots at either end,and generally have positive optical power and may include single ormultiple optical elements to provide such positive optical power.Conversely, concave lenses (represented by concave graphics) generallyhave negative optical power and may include single or multiple opticalelements to provide such negative optical power. The moving elements1122 may move along the optical axis to adjust the zoom of the image asdesired. Light from zoom 1112 then passes through a first z-screen 1124and then toward a screen 1150 to form a first image.

S-polarized light from PBS 1106 that is reflected toward the seconddirection passes through aperture stop 1110. Subsequently, the light isreflected by about 90 degrees by a reflecting element 1130, such as aright angle prism with mirror 1130. the s-polarized light then passesthrough second mechanically compensate afocal zoom 1132. Zoom 1132 mayemploy a similar structure and operate in a similar way to the structureand operation described for zoom 1112. Of course, the moving elements1134 may be adjusted differently, to provide a different zoom, asdesired. S-polarized light from zoom 1132 may then pass through rotator1136, which may be an achromatic half wave plate. Rotator 1136 functionsto rotate the s-polarized light into p-polarized light. The p-polarizedlight on the second light path then passes through second z-screen 1138,and then toward screen 1150, to form a second image. The first andsecond images are overlaid at screen 1150.

The following discussion relates to further embodiments, components usedin the disclosed embodiments, and variations of embodiments disclosedherein.

Polarizing beam splitter: The exemplary PBS shown in FIG. 3 through FIG.12 is depicted as a PBS plate. This PBS plate may be constructed using awire grid layer on glass (e.g. Proflux polarizer from Moxtek in Orem,Utah, polarization recycling film (e.g., Double Brightness EnhancingFilm from 3M in St. Paul, Minn.), polarization recycling film on glass(for flatness), or a multi-dielectric layer on glass. The PBS could alsobe implemented as a glass cube (with wire grid, polarization recyclingfilm, or dielectric layers along the diagonal).

Adjustment of image location: In FIG. 3, the primary adjustment of imagelocation for each path is lateral displacement of the projection lenses328 and 330. Additional adjustment of the image location may be achievedby adjusting the PBS 310 and/or the mirror 318. In FIG. 4 and FIG. 5,the primary adjustment of the image location for each path is thelateral displacement of the projection lenses 428/430 and 528/530.Additional fine adjustment of the image location may be achieved bylaterally displacing and tilting the prism structure 402. In FIG. 6 andFIG. 7, adjustment of the image overlay can be achieved by fineadjustment of the prism location and tilt. Adjustment of the imagelocation on-screen may be accomplished by lateral displacement of theprojection lens (630 or 730). In FIG. 8 through FIG. 10, adjustment ofthe image overlay may be accomplished by tilting the polarizing beamsplitter (810, 910, or 1010) and/or tilting the prism (814, 914, or1014). Adjustment of the aforementioned components (PBS, Mirror and/orprojection lenses) to control image location may be accomplished usingelectro-mechanical actuators. Feedback control systems and sensors mayprovide processing, control and drive instructions to the actuators inorder to position the location of the first and second images on thescreen 102.

Polarization switch: The polarization switch, as illustrated indisclosed embodiments, may be a circular polarization switch or a linearpolarization switch (e.g., Z-screen of U.S. Pat. No. 4,792,850 toLipton, or one of the Achromatic Polarization Switches as disclosed inU.S. patent application Ser. No. 11/424,087, all of which are previouslyy incorporated by reference). Another technique disclosed herein forswitching polarization includes using a rotating polarization wheel, asshown in the embodiments taught with reference to FIGS. 5, 7, 9 and 10.For that matter, the polarization switch 312 can be any switch thatalternates between orthogonal polarization states, such that the eyewear104 can decode the states and send the appropriate imagery to each eye.The polarization switch can be split between the two paths (e.g. toincrease yield of the device).

Transmission and stray light control: All transmissive elements may beanti-reflection coated to provide high transmission and low reflection.Reflections from transmissive elements can cause stray light in thesystem, which degraded contrast and/or produces disturbing artifacts inthe final image. Non-optical surface (e.g., the prism sides) can bepainted black to enhance contrast. Additional absorptive polarizers maybe placed after the PBS 310 in either path to control polarizationleakage and improve the final image contrast.

Fold mirror and polarization purity: The fold mirror may be replacedwith a PBS element (e.g., wire grid plate) in FIG. 3 through FIG. 10. Inthis case, a purer polarization may be maintained after the foldingelement and could negate the need for an input polarizer on thepolarization switch. Additionally, light reflected at the angled face ofthe prism may use total internal reflection for the reflectingmechanism. Dielectric and metal layers may also be added to the prism atthe angled face to enhance reflection and preserve polarization purity.

Projection Lenses: Although the embodiments of FIGS. 3-10 illustrate theuse of projection lenses with reverse telephoto construction, thepolarization conversion systems disclosed herein are not limited tousing such projection lenses. A reverse telephoto lens in a compact formis described in U.S. Pat. No. 6,473,242 ('242 patent), which is herebyincorporated by reference. For instance, FIG. 12 illustrates a tenthembodiment of the polarization conversion system 1200 that provides apolarization beam splitter internal to the projection lens, differingfrom the reverse telephoto lens design of the '242 patent. In thisembodiment, the polarizing beam splitter 1210 is incorporated into thelens (1230 a, 1230 b and 1230 c) at the aperture stop, and two opticalpaths 1212 and 1214 exist for overlaying the two polarization states outof the projector. In this example, the mirror 1216, rotator 1218 andpolarization switches 1220 and 1222 are located after the second half ofthe projection lens (1230 b and 1230 c), between the lens 1230 andsilver screen. The lens proscription has been modified to producecollimated rays from each field point at the aperture stop. Thismodification results two particular differences from the lens describedin the '242 patent. First, whereas the lens of the '242 patent satisfiesthe conciliation “0.18<r4/f<0.45,” the lens described herein has norestriction on r4 (e.g., r4/f could be a 0.6 in this instance). Second,whereas the lens of the '242 patent includes a “third lens group havinga positive refraction power,” the lens described herein may include athird lens having negative refractive power. As a consequence of themodification, the lens described herein is no longer reverse telephoto.A PBS 1210, mirror 1216, and polarization switch(es) 1220, 1222 areincluded for the PCS function. The mirror 1216 can be tilted to alignthe two images at the screen. In some embodiments, a right-angle glassprism may substitute the mirror 1216. In some embodiments, the PBS 1210can be replaced with a PBS cube. In the diagram, the polarizationswitches are placed at the output of the lens for highest polarizationpurity. One or two polarization switches may be used at the output. Onepath may include an achromatic rotator prior to the switch.

While various embodiments in accordance with the principles disclosedherein have been described above, it should be understood that they havebeen presented by way of example only, and not limitation. Thus, thebreadth and scope of the invention(s) should not be limited by any ofthe above-described exemplary embodiments, but should be defined only inaccordance with any claims and their equivalents issuing from thisdisclosure. Furthermore, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganization cues. These headings shall not limit or characterize theinvention(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called field. Further, adescription of a technology in the “Background” is not to be construedas an admission that certain technology is prior art to any invention(s)in this disclosure. Neither is the “Brief Summary” to be considered as acharacterization of the invention(s) set forth in issued claims.Furthermore, any reference in this disclosure to “invention” in thesingular should not be used to argue that there is only a single pointof novelty in this disclosure. Multiple inventions may be set forthaccording to the limitations of the multiple claims issuing from thisdisclosure, and such claims accordingly define the invention(s), andtheir equivalents, that are protected thereby. In all instances, thescope of such claims shall be considered on their own merits in light ofthis disclosure, but should not be constrained by the headings set forththerein.

1-20. (canceled)
 21. A polarization conversion system for transmittingpolarization encoded imagery to a projection screen, comprising: apolarization beam splitter operable to receive image light from an imagesource, operable to transmit image light of a first polarization statetoward a first image light path, and operable to reflect image light ofa second polarization state toward a second image light path; a firstpolarization switch located on at least one of the first image lightpath and the second image light path, the polarization switch operableto selectively alternate the polarization of image light passingtherethrough between a first output state of polarization and a secondoutput state of polarization so that the image light passingtherethrough has sequentially alternating polarizations; a firstprojection lens located on the first image light path, wherein the firstprojection lens is operable to project the image light of the firstimage light path onto the projection screen as a first diverging lightbeam between the first projection lens and the projection screen; asecond projection lens located on the second image light path, whereinthe second projection lens is operable to project the image light of thesecond path image light onto the projection screen as a second diverginglight beam between the second projection lens and the projection screen;wherein the first and second projection lenses are further operable todirect the first diverging light beam and the second diverging lightbeam to superimpose and substantially overlap with each other on theprojection screen; wherein the polarization conversion system ispositioned between the image source and the projection screen; andwherein the first polarization switch operates such that the first andsecond diverging light beams have the same state of polarization as theysimultaneously reach the projection screen.
 22. The polarizationconversion system of claim 21, wherein the image light received atpolarization beam splitter is randomly polarized light.
 23. Thepolarization conversion system of claim 21, wherein nearly all of thelight received at the polarization beam splitter is imaged at theprojection screen with a single polarization state.
 24. The polarizationconversion system of claim 21, further comprising a reflector positionedon the second image light path, wherein the reflector is operable withthe second projection lens to direct the second image light from thesecond image light path to be superimposed on and substantially overlapon the projection screen with the first image light from the first imagelight path.
 25. The polarization conversion system of claim 24, whereinthe reflector is a mirror.
 26. The polarization conversion system ofclaim 24, wherein the reflector is a prism.
 27. The polarizationconversion system of claim 21, further comprising a relay lens locatedoptically after the first polarization switch.
 28. The polarizationconversion system of claim 21, further comprising a polarization rotatorlocated optically before the first polarization switch.
 29. Thepolarization conversion system of claim 28, wherein the polarizationrotator is a half-wave plate.
 30. The polarization conversion system ofclaim 28, wherein the polarization rotator is optically adjacent to thefirst polarization switch.
 31. The polarization conversion system ofclaim 21, further comprising a second polarization switch located on theother of the first image light path and the second image light pathrelative to the first polarization switch.
 32. The polarizationconversion system of claim 31, further comprising a relay lens locatedoptically after the second polarization switch
 33. The polarizationconversion system of claim 31, further comprising a polarization rotatorlocated optically before the second polarization switch.
 34. Thepolarization conversion system of claim 33, wherein the polarizationrotator is a half-wave plate.
 35. The polarization conversion system ofclaim 33, wherein the polarization rotator is optically adjacent to thesecond polarization switch.
 36. The polarization conversion system ofclaim 21, wherein the first polarization switch is located opticallybefore the first or the second projection lens on the respective firstor second image light path.
 37. The polarization conversion system ofclaim 21, wherein the first polarization switch is located opticallyafter the first or the second projection lens on the respective first orsecond image light path.
 38. The polarization conversion system of claim31, wherein the second polarization switch is located optically beforethe first or the second projection lens on the respective first orsecond image light path.
 39. The polarization conversion system of claim31, wherein the second polarization switch is located optically afterthe first or the second projection lens on the respective first orsecond image light path.
 40. The polarization conversion system of claim21, further comprising a prism positioned on the second image lightpath, wherein the prism is operable with the second projection lens todirect the second image light from the second image light path to besuperimposed on and substantially overlap on the projection screen withthe first image light from the first image light path.
 41. Thepolarization conversion system of claim 40, wherein the prism is a glassprism.
 42. The polarization conversion system of claim 40, wherein theprism comprises an angled surface that optically operates as a foldmirror.
 43. The polarization conversion system of claim 42, wherein theangled surface operates to reflect the image light striking it throughtotal internal reflection.
 44. The polarization conversion system ofclaim 42, wherein the angled surface is coated with a mirror layer. 45.The polarization conversion system of claim 44, wherein the coatedmirror layer comprises silver.
 46. The polarization conversion system ofclaim 40, further comprising an aperture stop optically before theprism.
 47. The polarization conversion system of claim 40 wherein theprism is operable to be tilted in order to facilitate the directing ofthe first diverging light beam and the second diverging light beam tosuperimpose and substantially overlap with each other on the projectionscreen.
 48. The polarization conversion system of claim 21 wherein thepolarizing beam splitter is operable to be tilted in order to facilitatethe directing of the first diverging light beam and the second diverginglight beam to superimpose and substantially overlap with each other onthe projection screen.
 49. The polarization conversion system of claim21 wherein the projection lenses are operable to be laterally displacedfrom the light path in order to facilitate the directing of the firstdiverging light beam and the second diverging light beam to superimposeand substantially overlap with each other on the projection screen. 50.The polarization conversion system of claim 21, wherein the firstpolarization switch operates such that the first and second diverginglight beams have the same states of polarization as they simultaneouslyreach the projection screen, and wherein the states of polarizationalternative between horizontal and vertical linear polarization.
 51. Thepolarization conversion system of claim 21, wherein the firstpolarization switch operates such that the first and second diverginglight beams have the same states of polarization as they simultaneouslyreach the projection screen, and wherein the states of polarizationalternative between left- and right-handed circular polarization.
 52. Apolarization conversion system for transmitting polarization encodedimagery to a projection screen, comprising: a polarization beam splitteroperable to receive image light from an image source, operable totransmit image light of a first polarization state toward a first imagelight path, and operable to reflect image light of a second polarizationstate toward a second image light path; a first polarization switchlocated on at least one of the first image light path and the secondimage light path, the polarization switch operable to selectivelyalternate the polarization of image light passing therethrough between afirst output state of polarization and a second output state ofpolarization so that the image light passing therethrough hassequentially alternating polarizations; a first projection lens locatedon the first image light path, wherein the first projection lens isoperable to project the image light of the first image light path ontothe projection screen as a first diverging light beam between the firstprojection lens and the projection screen; a second projection lenslocated on the second image light path, wherein the second projectionlens is operable to project the image light of the second path imagelight onto the projection screen as a second diverging light beambetween the second projection lens and the projection screen; and areflector positioned on the second image light path, wherein thereflector is operable with the second projection lens to direct thesecond image light from the second image light path to be superimposedon and substantially overlap on the projection screen with the firstimage light from the first image light path; wherein the image lightreceived at polarization beam splitter is randomly polarized light;wherein the first and second projection lenses are further operable todirect the first diverging light beam and the second diverging lightbeam to superimpose and substantially overlap with each other on theprojection screen; wherein the polarization conversion system ispositioned between the image source and the projection screen; whereinthe first polarization switch operates such that the first and seconddiverging light beams have the same state of polarization as theysimultaneously reach the projection screen; and wherein nearly all ofthe light received at the polarization beam splitter is imaged at theprojection screen with a single polarization state
 53. The polarizationconversion system of claim 52, wherein the reflector is a mirror. 54.The polarization conversion system of claim 52, wherein the reflector isa prism.
 55. The polarization conversion system of claim 52, furthercomprising a polarization rotator located optically before the firstpolarization switch.
 56. The polarization conversion system of claim 52,wherein the polarization rotator is a half-wave plate.
 57. Thepolarization conversion system of claim 52, wherein the polarizationrotator is optically adjacent to the first polarization switch.
 58. Thepolarization conversion system of claim 52, further comprising a secondpolarization switch located on the other of the first image light pathand the second image light path relative to the first polarizationswitch.
 59. The polarization conversion system of claim 58, furthercomprising a polarization rotator located optically before the secondpolarization switch.
 60. The polarization conversion system of claim 58,wherein the polarization rotator is a half-wave plate.
 61. Thepolarization conversion system of claim 58, wherein the polarizationrotator is optically adjacent to the second polarization switch.
 62. Thepolarization conversion system of claim 52, wherein the firstpolarization switch is located optically before the first or the secondprojection lens on the respective first or second image light path orafter the first or the second projection lens on the respective first orsecond image light path.
 63. The polarization conversion system of claim58, wherein the second polarization switch is located optically beforethe first or the second projection lens on the respective first orsecond image light path.
 64. The polarization conversion system of claim58, wherein the second polarization switch is located optically afterthe first or the second projection lens on the respective first orsecond image light path.
 65. The polarization conversion system of claim52, further comprising a prism positioned on the second image lightpath, wherein the prism is operable with the second projection lens todirect the second image light from the second image light path to besuperimposed on and substantially overlap on the projection screen withthe first image light from the first image light path.
 66. Thepolarization conversion system of claim 65, further comprising anaperture stop optically before the prism.
 67. The polarizationconversion system of claim 65, wherein the prism is operable to betilted in order to facilitate the directing of the first diverging lightbeam and the second diverging light beam to superimpose andsubstantially overlap with each other on the projection screen.
 68. Thepolarization conversion system of claim 52, wherein the firstpolarization switch operates such that the first and second diverginglight beams have the same states of polarization as they simultaneouslyreach the projection screen, and wherein the states of polarizationalternative between horizontal and vertical linear polarization.
 69. Apolarization conversion system for transmitting polarization encodedimagery to a projection screen, comprising: a polarization beam splitteroperable to receive image light from an image source, operable totransmit image light of a first polarization state toward a first imagelight path, and operable to reflect image light of a second polarizationstate toward a second image light path; a first polarization switchlocated on at least one of the first image light path and the secondimage light path, the polarization switch operable to selectivelyalternate the polarization of image light passing therethrough between afirst output state of polarization and a second output state ofpolarization so that the image light passing therethrough hassequentially alternating polarizations; a first projection lens locatedon the first image light path, wherein the first projection lens isoperable to project the image light of the first image light path ontothe projection screen as a first diverging light beam between the firstprojection lens and the projection screen; a second projection lenslocated on the second image light path, wherein the second projectionlens is operable to project the image light of the second path imagelight onto the projection screen as a second diverging light beambetween the second projection lens and the projection screen; and apolarization rotator located optically before the first polarizationswitch; wherein the image light received at polarization beam splitteris randomly polarized light, wherein the first and second projectionlenses are further operable to direct the first diverging light beam andthe second diverging light beam to superimpose and substantially overlapwith each other on the projection screen; wherein the polarizationconversion system is positioned between the image source and theprojection screen; wherein the first polarization switch operates suchthat the first and second diverging light beams have the same state ofpolarization as they simultaneously reach the projection screen; andwherein nearly all of the light received at the polarization beamsplitter is imaged at the projection screen with a single polarizationstate.
 70. The polarization conversion system of claim 69, wherein thepolarization rotator is a half-wave plate.
 71. The polarizationconversion system of claim 69, wherein the polarization rotator isoptically adjacent to the first polarization switch.
 72. Thepolarization conversion system of claim 69, further comprising areflector positioned on the second image light path, wherein thereflector is operable with the second projection lens to direct thesecond image light from the second image light path to be superimposedon and substantially overlap on the projection screen with the firstimage light from the first image light path.
 73. The polarizationconversion system of claim 72, wherein the reflector is a mirror. 74.The polarization conversion system of claim 72, wherein the reflector isa prism.
 75. The polarization conversion system of claim 69, wherein thefirst polarization switch is located optically before the first or thesecond projection lens on the respective first or second image lightpath or after the first or the second projection lens on the respectivefirst or second image light path.
 76. The polarization conversion systemof claim 69, further comprising a prism positioned on the second imagelight path, wherein the prism is operable with the second projectionlens to direct the second image light from the second image light pathto be superimposed on and substantially overlap on the projection screenwith the first image light from the first image light path.
 77. Apolarization conversion system for transmitting polarization encodedimagery to a projection screen, comprising: a polarization beam splitteroperable to receive image light from an image source, operable totransmit image light of a first polarization state toward a first imagelight path, and operable to reflect image light of a second polarizationstate toward a second image light path; a first polarization switchlocated on at least one of the first image light path and the secondimage light path, the polarization switch operable to selectivelyalternate the polarization of image light passing therethrough between afirst output state of polarization and a second output state ofpolarization so that the image light passing therethrough hassequentially alternating polarizations; a first projection lens locatedon the first image light path, wherein the first projection lens isoperable to project the image light of the first image light path ontothe projection screen as a first diverging light beam between the firstprojection lens and the projection screen; a second projection lenslocated on the second image light path, wherein the second projectionlens is operable to project the image light of the second path imagelight onto the projection screen as a second diverging light beambetween the second projection lens and the projection screen; apolarization rotator located optically before the first polarizationswitch a reflector positioned on the second image light path, whereinthe reflector is operable with the second projection lens to direct thesecond image light from the second image light path to be superimposedon and substantially overlap on the projection screen with the firstimage light from the first image light path wherein the image lightreceived at polarization beam splitter is randomly polarized light;wherein the first and second projection lenses are further operable todirect the first diverging light beam and the second diverging lightbeam to superimpose and substantially overlap with each other on theprojection screen; wherein the polarization conversion system ispositioned between the image source and the projection screen; whereinthe first polarization switch operates such that the first and seconddiverging light beams have the same state of polarization as theysimultaneously reach the projection screen; and wherein nearly all ofthe light received at the polarization beam splitter is imaged at theprojection screen with a single polarization state.
 78. The polarizationconversion system of claim 77, wherein the reflector is a mirror. 79.The polarization conversion system of claim 77, wherein the reflector isa prism.
 80. The polarization conversion system of claim 77, wherein thepolarization rotator is a half-wave plate.
 81. The polarizationconversion system of claim 77, wherein the polarization rotator isoptically adjacent to the first polarization switch.
 82. Thepolarization conversion system of claim 77, further comprising a secondpolarization switch located on the other of the first image light pathand the second image light path relative to the first polarizationswitch.
 83. The polarization conversion system of claim 82, wherein thefirst polarization switch is located optically before the first or thesecond projection lens on the respective first or second image lightpath or after the first or the second projection lens on the respectivefirst or second image light path.
 84. The polarization conversion systemof claim 82, wherein the second polarization switch is located opticallybefore the first or the second projection lens on the respective firstor second image light path or after the first or the second projectionlens on the respective first or second image light path.
 85. Thepolarization conversion system of claim 77, further comprising a prismpositioned on the second image light path, wherein the prism is operablewith the second projection lens to direct the second image light fromthe second image light path to be superimposed on and substantiallyoverlap on the projection screen with the first image light from thefirst image light path.
 86. The polarization conversion system of claim85, further comprising an aperture stop optically before the prism. 87.The polarization conversion system of claim 77, wherein the polarizingbeam splitter is operable to be tilted in order to facilitate thedirecting of the first diverging light beam and the second diverginglight beam to superimpose and substantially overlap with each other onthe projection screen.
 88. The polarization conversion system of claim77, wherein the projection lenses are operable to be laterally displacedfrom the light path in order to facilitate the directing of the firstdiverging light beam and the second diverging light beam to superimposeand substantially overlap with each other on the projection screen. 89.The polarization conversion system of claim 77, wherein the firstpolarization switch operates such that the first and second diverginglight beams have the same states of polarization as they simultaneouslyreach the projection screen, and wherein the states of polarizationalternative between horizontal and vertical linear polarization.
 90. Thepolarization conversion system of claim 77, wherein the firstpolarization switch operates such that the first and second diverginglight beams have the same states of polarization as they simultaneouslyreach the projection screen, and wherein the states of polarizationalternative between left- and right-handed circular polarization.
 91. Apolarization conversion system for transmitting polarization encodedimagery to a projection screen, comprising: a polarization beam splitteroperable to receive image light from an image source, operable totransmit image light of a first polarization state toward a first imagelight path, and operable to reflect image light of a second polarizationstate toward a second image light path; a first polarization switchlocated on at least one of the first image light path and the secondimage light path, the polarization switch operable to selectivelyalternate the polarization of image light passing therethrough between afirst output state of polarization and a second output state ofpolarization so that the image light passing therethrough hassequentially alternating polarizations; a first projection lens locatedon the first image light path, wherein the first projection lens isoperable to project the image light of the first image light path ontothe projection screen as a first diverging light beam between the firstprojection lens and the projection screen; a second projection lenslocated on the second image light path, wherein the second projectionlens is operable to project the image light of the second path imagelight onto the projection screen as a second diverging light beambetween the second projection lens and the projection screen; a secondpolarization switch located on the other of the first image light pathand the second image light path relative to the first polarizationswitch; a polarization rotator located optically before the secondpolarization switch; and wherein the image light received atpolarization beam splitter is randomly polarized light; wherein thefirst and second projection lenses are further operable to direct thefirst diverging light beam and the second diverging light beam tosuperimpose and substantially overlap with each other on the projectionscreen; wherein the polarization conversion system is positioned betweenthe image source and the projection screen; wherein the firstpolarization switch operates such that the first and second diverginglight beams have the same state of polarization as they simultaneouslyreach the projection screen; and wherein nearly all of the lightreceived at the polarization beam splitter is imaged at the projectionscreen with a single polarization state.
 92. The polarization conversionsystem of claim 91, further comprising a reflector positioned on thesecond image light path, wherein the reflector is operable with thesecond projection lens to direct the second image light from the secondimage light path to be superimposed on and substantially overlap on theprojection screen with the first image light from the first image lightpath.
 93. The polarization conversion system of claim 92, wherein thereflector is a mirror.
 94. The polarization conversion system of claim92, wherein the reflector is a prism.
 95. The polarization conversionsystem of claim 91, wherein the polarization rotator is a half-waveplate.
 96. The polarization conversion system of claim 91, wherein thepolarization rotator is optically adjacent to the second polarizationswitch.
 97. The polarization conversion system of claim 91, wherein thefirst polarization switch is located optically before the first or thesecond projection lens on the respective first or second image lightpath or after the first or the second projection lens on the respectivefirst or second image light path.
 98. The polarization conversion systemof claim 91, wherein the second polarization switch is located opticallybefore the first or the second projection lens on the respective firstor second image light path or after the first or the second projectionlens on the respective first or second image light path.
 99. Thepolarization conversion system of claim 91, further comprising a prismpositioned on the second image light path, wherein the prism is operablewith the second projection lens to direct the second image light fromthe second image light path to be superimposed on and substantiallyoverlap on the projection screen with the first image light from thefirst image light path.
 100. The polarization conversion system of claim91, wherein the first polarization switch operates such that the firstand second diverging light beams have the same states of polarization asthey simultaneously reach the projection screen, and wherein the statesof polarization alternative between horizontal and vertical linearpolarization.
 101. The polarization conversion system of claim 91,wherein the first polarization switch operates such that the first andsecond diverging light beams have the same states of polarization asthey simultaneously reach the projection screen, and wherein the statesof polarization alternative between left- and right-handed circularpolarizat